Oil or gas wells are often surveyed to determine one or more geological, petrophysical, geophysical, and well production properties (“parameters of interest”) using electronic measuring instruments conveyed into the borehole by an umbilical such as a cable, a wireline, slickline, drill pipe or coiled tubing. Tools adapted to perform such surveys are commonly referred to as formation evaluation tools. These tools use electrical, acoustical, nuclear and/or magnetic energy to stimulate the formations and fluids within the borehole and measure the response of the formations and fluids. The measurements made by downhole instruments are transmitted back to the surface. In many instances, multiple trips or logging runs are needed to collect the necessary data. Additionally, the logging speed is usually a predetermined fixed quantity.
In order to reduce the amount of rig time needed for wireline logging, it is common practice to run multiple sensors in a single run. FOCUS™, from Baker Atlas Inc., is an open hole logging systems suitable for use with the present invention. All of the downhole instruments have been redesigned, incorporating advanced downhole sensor technology, into shorter, lighter, more reliable logging instruments, capable of providing formation evaluation measurements with the same precision and accuracy as the industry's highest quality sensors, at much higher logging speeds. Logging speeds are up to twice the speed of conventional triple-combo and quad combo logging tool strings. Speeds of up to 3600 ft/hr (1080 m/min) are possible. The logging system may include four standard major open hole measurements (resistivity, density, neutron, acoustic) plus auxiliary services.
The resolution and accuracy of logging measurements is determined by the type of measurement and the type of formation being logged. The measurement may be tailored to the type of formation. For example, U.S. Pat. No. 5,309,098 to Coates et al. teaches a method and apparatus in which a variable time-window echo-recording system is used to obtain significant improvements in signal quality and logging speed for NMR measurements. An initial test is performed to provide an assessment of the relaxation qualities of the sample. If the test reveals that the sample is a slow-relaxation rock, then the full time is allocated to measuring echoes. However, if the test reveals that the sample is a fast decay rock, then the echo acquisition time window is reduced. This provides increased efficiency since the system is able to maximize the number of measurements made by optimizing the individual sampling intervals to the particular geologic structure being tested.
Generally, prior art methods have conducted logging at a uniform logging speed. A fixed logging speed is used for the entire logging interval. This flies in the face of logic since reservoir intervals form only a small portion of the entire geologic section and it is only in reservoir intervals is it necessary to get precise and accurate measurements with high resolution: in the non-reservoir intervals, high precision and accuracy are not usually necessary.
It would be desirable to have a method and apparatus of logging a borehole in which the inefficiencies of the prior art are overcome. Such an invention should preferably be able to accommodate a variety of logging tools. The present invention satisfies this need.